A series of optically active cis-3 bis-adducts, such as (R,R,fC)-16 (Scheme 6), was obtained regio- and diastereoselectively by Bingel macrocyclization of C60 with bis-malonates, which contain optically active tethers derived from 1,2-diols. The absolute configuration of the inherently chiral addition pattern in cis-3 bis-adducts had previously been determined by comparison of calculated and experimental circular dichroism (CD) spectra. Full confirmation of these earlier assignments was now obtained by an independent method based on semiempirical AM1 (`Austin Model 1') and OM2 (`Orthogonalization Method 2') calculations combined with 1H-NMR spectroscopy. It was found computationally that bis-malonates [CHR(OCOCH2COOEt)]2, which contain (R,R)- or (S,S)-butane-2,3-diol derivatives as optically active tethers, preferentially form out-out cis-3 bis-adducts of C60 as a single diastereoisomer in which the alkyl groups R adopt a gauche conformation, while the two glycolic H-atoms are in an antiperiplanar (ap) and the ester linkages to the fullerene in a gauche relationship (Figs. 2 and 5). In contrast, in the less favorable diastereoisomer, which should not form, the alkyl groups R adopt an ap and the H-atoms a gauche conformation, while the ester bridges to the fullerene remain, for geometric reasons, locked in a gauche conformation. According to the OM2 calculations, the geometry of the fully staggered tether in the free bis-malonates closely resembles the conformation of the tether fragment in the bis-adducts formed. These computational predictions were confirmed experimentally by the measurement of the coupling constant between the vicinal glycolic H-atoms in the 1H-NMR spectrum. For (R,R,fC)-16, 3J(H,H) was determined as 7.9 Hz, in agreement with the ap conformation, and, in combination with the calculations, this allowed assignment of the fC-configuration to the inherently chiral addition pattern. This conformational analysis was further supported by the regio- and diastereoselective synthesis of cis-3 bis-adducts from bis-malonates, including tethers derived from cyclic glycol units with a fixed gauche conformation of the alkyl residues R at the glycolic C-atoms. Thus, a bis-malonate of (R,R)-cyclohexane-1,2-diol provided exclusively cis-3 bis-adduct (R,R,fC)-20 in 32% yield (Scheme 7). Incorporation of a tether derived from methyl 4,6-O,O-benzylidene-α-D-glucopyranoside into the bis-malonate and Bingel macrocyclization diastereoselectively produced the cis-3 stereoisomer (α,D,fA)-22 (Scheme 8) as the only macrocyclic bis-adduct. If the geometry of the alkyl groups R at the glycolic C-atoms of the tether component deviates from a gauche relationship, as in the case of tethers derived from exo cis- and trans-norbornane-2,3-diol or from trans-cyclopentane-1,2-diol, hardly any macrocyclic product is formed (Schemes 5 and 9). The absolute configurations of the various optically active cis-3 bis-adducts were also assigned by comparison of their CD spectra, which are dominated by the chiroptical contributions of the inherently chiral fullerene chromophore (Figs. 1, 3, and 4). A strong chiral exciton coupling was observed for optically active macrocyclic cis-3 bis-adducts of C60 with two appended 4-(dimethylamino)benzoate ((S,S,fC)-26; Fig. 6) or meso-tetraphenylporphyrin ((R,R,fC)-28; Fig. 7) chromophores. Chiral exciton coupling between two fullerene chromophores was observed for the first time in the CD spectrum of the threitol-bridged bis-fullerene (R,R)-35 (Fig. 9).
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Jun 19, 2013
Kang Dai + 2
Open Access